The present invention generally relates to thermoplastic materials. Some particular embodiments of the invention are directed to copolycarbonates having improved flow properties under industrial molding conditions.
Thermoplastic polymers have many desirable properties that make them useful in molding applications. Polycarbonates are excellent examples of such materialsxe2x80x94especially polycarbonates based on bisphenol A (BPA). These resins possess properties such as strength, toughness, heat resistance, chemical resistance, and optical clarity. They can also be modified or blended to enhance their melt flow characteristics. Thus, polycarbonates are often the materials of choice for many injection molding applications.
One such application of great interest over the last decade is optical media, e.g., compact discs (CD""s) and digital versatile discs (DVD""s). This type of media contains optically-encoded digital information. The physical structure of the disc is usually composed of polycarbonate material. The disc is fabricated by injecting-molding molten polycarbonate into a cavity where micro-pits are formed. The micro-pits represent encoded information, and are usually arranged in a spiral data track. The side where the micro pit track has been formed is coated with a thin layer of deposited metal (e.g., aluminum), to make the surface reflective.
The polycarbonate material serves as both a support structure and an optical window, through which a laser is focused on the reflective layer containing the data tracks. Each pit in the track varies in length according to the encoded digital data. Depending on whether the laser light is scattered or reflected back is related to pit presence and length, and provides the basis for recovering a digital data stream which represents the desired recorded information.
Electronics companies continually strive to incorporate greater amounts of information into optical discs. New formats to satisfy this need have been developed and refined, such as DVD-recordable (DVD-R), DVD-rewritable, high-density DVD, and digital video-recordable (DVR). The formats may include multiple information layers and/or shorter-wavelength lasers. The discs themselves must continue to reliably possess certain attributes, such as dimensional stability, high replication (i.e., precise replication from a stamper), and minimal birefringence levels.
In some instances, BPA polycarbonates do not fully meet the requirements for current disc formats, or for discs being developed for next-generation optical storage. For example, these conventional polycarbonates typically have high melt viscosities, which can impede melt flow. Insufficient melt flow during molding (e.g., injection molding) can lead to decreased pit or groove replication and/or longer molding cycle times. Moreover, while BPA polycarbonates do not typically absorb a great deal of water or moisture, they still may not meet the lower water-absorption requirements for more advanced optical disc applications.
It should therefore be apparent that some improvement in polycarbonate compositions would be welcome in the art. The compositions should exhibit lower viscosities at molding temperatures, as compared to conventional, bisphenol A-type polycarbonate compositions. Moreover, there should preferably be some improvement in terms of low water absorption. Furthermore, these improvements should be accompanied by the retention of other physical properties for the compositions, such as good impact strength, relatively high glass transition temperatures, and optical clarity.
A primary embodiment of the present invention is directed to a thermoplastic composition, comprising a copolycarbonate, i.e., a polycarbonate copolymer. The copolymer includes at least two structural units, I and II, which are described in detail below. In some preferred embodiments, structure I is a bisphenol A carbonate. Structure II is a bisphenol carbonate which includes branched or linear groups extending from a bridging carbon atom or carbon group which connects two aromatic rings. In preferred embodiments, the copolymer includes at least about 30 molar percent of structural unit I, based on the total molar weight of I and II. The thermoplastic composition can also include a variety of other additives.
The copolycarbonates of this invention can be prepared by various processes. Examples include melt polymerization and interfacial polymerization. They can also be prepared by a reactive blending process, as described below.
As described herein, the claimed copolycarbonates exhibit very good melt flow, along with enhanced shear-thinning behavior. These characteristics improve the molding capabilities of the compositionsxe2x80x94especially for demanding applications such as optical disc fabrication. Moreover, the compositions substantially retain other attributes which are important for various end uses, e.g., low moisture uptake, good impact strength, relatively high glass transition temperatures, and optical clarity. Thus, articles made from the compositions (such as optical discs) constitute another aspect of this invention.
An additional embodiment of the invention is directed to a process for molding thermoplastic articles, comprising the following steps. In the first step of the process, a copolycarbonate resin is formed, comprising structural units (I) and (II). The copolycarbonate resin is then formed into an article by a molding process, such as injection molding.
Further details regarding the various features of this invention are found in the remainder of the specification.